1. Field of the Invention
The present invention relates to a transmission system for a manually powered transport apparatus, such as a bicycle or the like. The transmission system includes a sprocket cluster, including a plurality of transmission sprockets, and a changing mechanism provided for shifting a chain from one transmission sprocket to another, along the sprocket cluster, in response to selected movement of an operation cable. The transmission system also includes a case for housing the sprocket cluster, the changing mechanism and the chain.
2. Description of the Background Art
Some known transmission systems including a case, a sprocket cluster, and a changing mechanism have previously been patented. As one example of a previously known transmission system, the bicycle transmission system disclosed in Japanese Laid-Open Patent No. 2004-155280,for example, includes a plurality of transmission sprockets with different outer diameters, a derailleur, for changing a chain from one transmission sprocket to another among the transmission sprockets upon operation of a wire for transmission operation, and a case for housing the sprockets, the chain and the derailleur. The transmission sprockets of the system disclosed in this reference are arranged along a transmission sprocket axis, in the order of decreasing outer diameter from left to right. The derailleur of the transmission system disclosed in Japanese Laid-Open Patent No. 2004-155280 includes a derailleur axle which is arranged along the transmission sprocket axis direction and is fixed to the case, and a derailleur arm which supports a guide pulley and is supported by the derailleur axle so as to be swingable. Upon operation of a wire, to which an operation component is connected, the derailleur arm and guide pulley swing around the derailleur axle, and move in the axis direction. The operation component can move axially in a housing hole provided inside the derailleur axle. Further, the operation component is fixed to the derailleur arm and engaged with a pin inserted through a guide hole provided in the derailleur axle. Since the derailleur is housed inside of the case in the transmission system of the reference, the operations of linking the operation component to the wire, linking the wire to the pin by use of the operation component, and releasing these linkages are performed through an opening provided in the right side wall of the case. After these linking operations are finished, the opening is plugged with a cap.
In Japanese Patent Laid-Open No. 2004-155280 described above, among the plurality of transmission sprockets, a transmission sprocket which has the smallest outer diameter and is located closest to the opening in the axis direction, is located farthest from the opening, in the direction orthogonal to the axis direction and in the radial direction. In contrast, a largest diameter transmission sprocket, which is located closer in the radial direction to the opening than the smallest diameter transmission sprocket, is located farthest to the opening in the axis direction. For this reason, the distance between the opening and the chain becomes large, when the chain is wrapped around either the largest or smallest diameter transmission sprocket. Thus, the opening needs to be enlarged in order to provide oil to the chain therethrough. However, enlargement of the opening entails a reduction in the rigidity of the case, and therefore the case itself needs to be enlarged, or the thickness of the case needs to be increased, in order to ensure required rigidity. However, adoption of such measures is problematic to some extent, because it makes the case bigger, and thus increases the weight of the case.